The use of optical fibers produced from glass, plastic or synthetic fused silica, often called silica or quartz fiber, is well known. One area where fiber optic technology has been developed to play a significant role is for the detection or measurement of liquid levels. In this regard, and as is indicated in U.S. Pat. No. 5,072,617 granted to Weiss on Dec. 17, 1991, the detection of liquid level is one of many functions for which fiber optic sensors may be more suitable than their electrical counterparts, especially in noisy or hostile environments.
In some applications, it may be highly desirable to provide a continuous indication of pressure or liquid level. However, in many applications, it may be quite sufficient to detect pressure or liquid levels at discrete levels. By way of example, if it is desired to monitor the level of a crude oil or other liquid within a hold of a ship, then it may be sufficient to detect the fullness of the hold within increments of 25%. Continuous measurements together with the relatively complex design considerations and instrumentation required to effect such measurements over broad pressure ranges may be considered unnecessary and impractical.
The prior art includes a variety of fiber optic devices that are designed to react to external fluid pressures applied to the device, whether as the result of a rising liquid level or otherwise. One class of such devices relies upon the use of a flexible membrane or diaphragm that resiliently stretches in response to external pressure and, in so doing, modifies the transmission of optical signals being carried to an from the device by fiber optic lines. Advantageously, the operation of devices that incorporate a pressure sensitive diaphragm generally does not depend upon the transmission of optical signals through the fluid medium under scrutiny. Accordingly, the operation is independent of the optical properties of the fluid.
One example of a pressure sensitive device that utilizes a flexible diaphragm is disclosed in U.S. Pat. No. 5,408,546 granted to Slaker et al. on Apr. 18, 1995. In this case, an optical signal is transmitted within a cylindrical housing or probe from one end and reflected back from an opposite end. The reflected signal is then detected at the first end. An opening in the wall of the housing is covered by a flexible membrane or diaphragm that normally appears to bulge into the housing and into the path of a transmitted signal. As disclosed, the device is apparently designed and used for continuous pressure measurements which are negative with respect to an ambient pressure and it is not clear how it might be easily adapted to sense or detect increasing positive pressure in a switching mode. In any case, for a given housing and a given diaphragm, the operating characteristics would be largely fixed. It does not appear that such characteristics could be altered except by using a diaphragm having different resilient characteristics and/or by modifying the size of the diaphragm in relation to the housing. Further, if the device could be adapted to sense or detect positive pressures in a switching mode, the device would be a normally "on" switch, and it is not apparent how it might be modified to operate as a normally "off" switch if a normally "off" characteristic was desired.
The patent to Weiss, supra, is another example of a pressure sensitive device that utilizes a flexible diaphragm. Weiss discloses a fiber-optic liquid level sensor that measures the height of a column of liquid through the hydrostatic pressure produced by the column on the diaphragm. As in the case of Shaker et al., the design of Weiss is intended for continuous measurements. But, unlike Slaker et al. where the diaphragm bulges into the path of a transmitted optical signal, the diaphragm of Weiss is used as a reflector that always stands completely in the path of a transmitted signal. The angle of reflection varies depending upon the degree of bulge thereby varying the signal intensity as seen by an associated signal detector. Such an arrangement requires a reflective diaphragm and the careful maintenance of optical alignment between input and output fiber optic lines and the diaphragm. Further, as in the case of Slaker et al., the operating characteristics of the device would be largely fixed for a given diaphragm.
A primary object of the present invention is to provide a new and improved pressure sensitive optical device of the class that utilizes a flexible diaphragm and that is particularly suitable for operation as an optical switch having on and off switching states.
A further object of the present invention is to provide a pressure sensitive optical switch where, for a given diaphragm, differing switching thresholds can be achieved with minimal design variation.
A still further object of the present invention is to provide a pressure sensitive optical switch where, for a given diaphragm, the switch may be operated as a normally "on" switch or, with minimal design variation, as a normally "off" switch.
Yet another object of the present invention is to provide new and improved apparatus utilizing a plurality of such switches for monitoring the level of a liquid.